(1) Field of the Invention
The present invention relates to a frequency discriminator of an input signal and, more particularly frequency discriminator for discriminating a frequency of a horizontal synchronizing signal inputted to a multi-mode monitor providing various screen modes.
(2) Description of the Prior Art
Generally, the font number of a screen displayed on a monitor varies depending on whether the monitor is in a VGA Video Graphic Array mode, a super VGA mode, an IBM 8514A mode, or the like.
For example, in the case of the VGA mode, there are nearly three kinds, including 640.times.350, 640.times.400, and 640.times.480. In the case of the super VGA mode, there is 800.times.600. In the case of the IBM 8514A mode, there is 1024.times.768
In a multi-mode monitor providing various screen modes, to automatically control a raster of the screen and any other setting state according to the change of modes, a frequency of an inputted horizontal synchronizing signal has to be discriminated.
A type of technology for discriminating the frequency of an inputted signal such as the above kind is disclosed in the Korean Examined Publication NO. 89-8733 entitled "Circuit for discriminating frequency of supply voltage".
In the above invention, when comparing an output signal of a counter for ranking a supplied frequency of the supply voltage with that of the counter for ranking a reference frequency, if the frequency of the input supply voltage is more than 55 Hz, a signal of high state is outputted. If the frequency of the input supply voltage is less than 55 Hz, a signal of low state is outputted whereby the operation characteristic for discriminating whether the frequency of the input supply voltage is 50 Hz or 60 Hz can be achieved.
However, the above invention has a disadvantage in that, since only the resultant after discriminating whether the frequency of the input signal is larger or smaller than the reference frequency is outputted, while it can be applied to only two kinds of the frequency signals, it cannot be applied to various kinds of the frequency signals. This disadvantage leads to the inconvinence of not discriminating the various kinds of the frequency signals inputted to the multi-mode monitor providing the various modes.
Also, the above conventional discriminating circuit of the supply voltage frequency uses a sine wave signal as the input signal, so that it is difficult to apply a pulse signal to the multi-mode monitor as the input signal.
Another conventional frequency discriminating circuit uses a frequency to voltage converter circuit. An output voltage, varied according to the input signals, is obtained by the frequency to voltage converter circuit and applied to one input terminal of the comparator whose other terminal is supplied with the different reference voltage. Then, the frequency of the input signal is discriminated by the output of the comparator.
However, in the conventional technologies, as a slope is changed in the frequency vs voltage characteristic of the frequency to voltage invertor circuit, the reference voltage of each comparator has to be controlled. In that case, many controlling components may need to be added during production of the frequency discriminating device and also, aging occurs.